Researchers from UCSC and Lawrence Livermore National Laboratory
(LLNL) have invented a new method for controlling the emission
of carbon dioxide from power plants.

Researchers are looking at new ways to solve the problem
of carbon emissions at power plants. A major Central Coast
power plant is this one at Moss Landing.Photo: Louise Donahue

The technique, which mimics natural weathering processes,
converts carbon dioxide gas into soluble compounds that can
be disposed of in the oceans.

Any strategy for curbing the progress of global warming is
likely to include technology that captures and stores carbon
dioxide, a byproduct of burning fossil fuels that is widely
regarded as the leading contributor to global climate change.

About one third of U.S. carbon emissions comes from large point
sources such as power plants.

"It has become clear to me that in addition to ringing
alarm bells about the carbon dioxide problem, Earth scientists
need to also think about ways to help solve the problem, beyond
just leaving fossil fuels in the ground," said coinventor
Gregory Rau, a research scientist with UCSC's Institute of Marine
Sciences who also works at LLNL.

Rau and LLNL researcher Ken Caldeira developed the new carbon
sequestration method, called Accelerated Weathering of Limestone.
The U.S. Department of Energy, which sponsored their work, has
now patented the technology.

The process involves reacting carbon dioxide in the stream
of waste gas from a power plant with water and calcium carbonate
(limestone) or other carbonate compounds. Instead of carbon
dioxide emissions, the plant generates wastewater rich in soluble
bicarbonate ions, which can be released beneath the surface
of the ocean. Rau said he expects this would have little impact
on the ocean.

"Limestone weathering is one of the ways the Earth naturally
mitigates increases in atmospheric carbon dioxide," Rau
said. "But nature is slow. We propose to speed up the limestone
weathering reaction."

When carbon dioxide dissolves in water, the water becomes acidic,
which makes it corrosive to limestone. Dissolution of limestone
by carbonated water generates soluble bicarbonate ions. This
is the process by which rainwater erodes limestone cliffs and
creates limestone caves and sinkholes.

Because the waste-gas stream of a power plant has a high carbon
dioxide concentration, water acidification will be rapid and
will lead to an efficient dissolution of the limestone, Rau
said.

In one of the model systems devised by Rau and Caldeira, waste
gases bubble up through a slurry of water and limestone particles.
Water is constantly sprinkled onto the slurry, and wastewater
laden with bicarbonates is pumped out.

The bicarbonates in the wastewater are in equilibrium with
dissolved carbon dioxide. Direct contact with ambient air would
shift the equilibrium, resulting in the escape of carbon dioxide
back into the atmosphere. To prevent this, the wastewater must
be released below the ocean surface, where it can mix and be
diluted with ocean water before coming into contact with the
atmosphere.

Alternative techniques for carbon sequestration include capture
and purification of waste carbon dioxide, followed by direct
injection into the deep ocean or underground into depleted oil
fields or salt formations. But these methods are costly, and
deep ocean injection would not neutralize the acidifying potential
of carbon dioxide, Rau said.

Oceans absorb a large portion of the carbon dioxide present
in the atmosphere. Scientists expect that the acidification
of oceans resulting from increased atmospheric carbon dioxide
will have deleterious effects on marine life, especially corals,
mollusks, and other creatures that make their shells or skeletons
out of calcium carbonate.

But wastewaters from Accelerated Weathering of Limestone would
carry the waste carbon mostly in the form of dissolved bicarbonates,
which should minimize adverse effects and might even benefit
coral reefs, Rau said.

Coal-burning power plants already use limestone scrubbing to
remove sulfur dioxide from their smokestack emissions, he said.
It might be possible to combine sulfur and carbon dioxide removal
in one step or in successive steps. Alternatively, Accelerated
Weathering of Limestone might be more appropriate for power
plants that burn natural gas, a cleaner fuel that does not require
sulfur removal. In either case, the water needed for the sequestration
reaction could come from the vast quantities of water already
used as coolant, Rau said.

Accelerated Weathering of Limestone would be most cost effective
in gas-burning plants located close to water and limestone sources,
he said. These conditions are met by several power plants along
the Florida and California coasts, which could serve as testing
grounds for this carbon dioxide sequestration method.